Advice: Best Approach for a Skinny, Underpowered Rider

Well done on that performance improvement - wouldn't say you are underpowered !

To chase strong riders on the flat, start with an effective aero position - fairly low at the front, elbows in, shoulders dropped, anterior hip rotation, slight tortoise neck etc. But you need to be comfortable and able to deliver power like that, so you'll need a bike fit.

Go for a fitter who's done thousands of fits rather than fancy kit and an instruction book.  I was very impressed  with Scheritt Knoesen at the Bike Whisperer, who spent a long time checking my flexibility, symmetry, stance/shoes and watching me ride under strain. Slight changes to my fit, and I get fewer aches than before. A good fit makes the bike like an extension of your own body - just think the move and it happens.

You've identified the need for core strength plus flexibility - so get stuck in to those routines well before the fit.

The bike frame is very much secondary. Deep section wheels help.  I'm guessing you don't have wide shoulders? So switch to narrower bars to reduce your cross-section.  You may need a change of saddle when you rotate your hips forward - slotted will take the pressure off your soft tissue. A heavy bike won't be an issue on the flat - TT bikes are often heavy for example.

I'm 63kg.  No problemo with 60/90 deep section wheels,  up to 25rmph crosswinds when I ride a bit further out.

Bikewise, it's a good time to buy. End of season sales soon.  I found a SuperSix Evo with Tiagra for £760 and did all this. Picture here - click on newest first.

I am not far off you (176cm and 60kg) and would say similar to you. Last year I finally replaced my old bike frame after 41,000 miles and just now I am going comfortably well at a level I am happy at. The new one is a bit lighter and more aero but the main difference I can perceive based on experience of other bikes is that its custom geometry and fitted to me making it really smooth

I bought a 14.3 kg bike recently. I thought it was light but scales showed 14.3

That was marvelous. Easy to hop up gutters on. Responsive. Felt really great until I broke the seat.

At home my comparable bike that I assembled weighs 11.9 kg. Yet it doesn't feel as light although measured using the same scale and being 2+ kg lighter.
Balance.

Aero is really nice. IMO. Neither of them are aero but the heavier bike had deep dish rims.
An aero bike I was is a track bike with brakes and super easy to maintain velocity upon. I like it.
I think the gain is there because if I hang a handlebar bag I can't spin as fast.

Anyone got any numbers for how much difference an "aero" frame makes vs "non aero"? Find it hard to believe its anything significant, plus you are then resigned to hideous aesthetics. Slightly concerned by a 55kg rider riding such deep section wheels? For that money get something that looks stunning, weighs about 7.5kgs with mid sections wheels? Good luck with your hunt

A bike that fits you and feels great is the most important thing. I strongly recommend that you shop around and try a range of makes and models. You may discover one that hadn't even been on your initial list.

An aero bike doesn't gain much as the rider makes up ~80% of the drag but it's always 'on', so to speak. Being able to hold an aero position and tight-fitting clothing certainly makes a significant difference.

A lightweight bike won't gain you much on the hills, but this is not where you are struggling. It will make zero difference on the flat.

55kg is unusually low for someone your height (you're 10cm taller than me yet 6kg lighter and I'm not overweight) so maybe consider whether your diet/nutrition could be better.

In the end your ability to keep up on the flat is down to a combination of power output and, when in a group, being able to ride close enough to the wheel in front to benefit. The biggest gains will come from improving your power.

Adding my voice to the chorus:  start with a good bike fit done by a pro.  It'll cost but you'll know exactly what to look for and will be able to narrow the field.  Every penny spent there will pay huge dividends later.  I'd also say yes for the deep section aero rims but read up on how they behave when riding conditions are real vs wind tunnel at a narrow set of angles:  you have a slight build so you'll disproportionally feel any effect of crosswinds and changes of wind resistance.  I'm 182 cm and when I was really fit 3 years ago, weighed 67 kilo.  As in your case, had a hard time keeping up with the speed machines on the flat but when things started going up it was their turn to die while I coasted up.  A couple of wheelsets that I tried back then  were positively scary in crosswinds or during abrupt changes such as when you're riding and a truck passes.  I know it sounds stupid but that's how it was and it got tiring, especially when I'd already been riding 80-100 km.     With your budget you'll be able to phenomenally improve on what you have no matter what direction you go.

+1 on the bike fit. If you’re happy to advise your location then am sure some forum members could provide you with a recommendation.

Definitely +1 for the bike fit because then you can really search out the manufacturers who cater for your physique.

The problem is that when they shrink bikes they often mess up the geo - a head tube can only go so short before downtube and toptube merge

Light as possible on the frame plus aero wheels would be my vote. Fundamentally you will expend more effort going fast on the flat than a standard or heavy rider because you'll be more affected by the wind. My riding buddy is 5-7, and wiry (chiropractor) - he leaves me for dead on the climbs and I lose him on descents/flat

For your budget you're going to get a big weight reduction regardless of what you go for. We seem to be seeing aero and lightweight merging more and more with bikes like the Tarmac having a bit of both so you could go down that route. Deep section carbon wheels will improve you're speed when blatting along so they'd be high on my spec list above an out and out aero frame with slightly less speedy wheels.

You being the engine and the air brake at the same time is the most important bit though so as said, make sure you're as aero as you can be and then the reduced weight of you new bike will really  show gains.

I'd love to recommend a bike, but it's such a personal (and fashionable) thing that what I'd choose might not suit you. Happy shopping 

Given your un-average physical characteristics, I would be tempted to specialise in climbing. It's not all about weight though obviously that helps, but frame geometry, stiffness and just how the response to input makes you feel. It sounds like your local hilly terrain should suit you down to the ground as you become the annoying git who crucifies the heavier more powerful riders 3/4 the way up a decent incline. But then again rides of up to 200Km would put comfort as a more important priority and maybe you don't want to be a git as much as I do.

Obviously what you need is at least 2 bikes!

Tbh most modern aero bikes are a decent (light) weight these days and you wont be losing much to a 'proper' lightweight climber...and certainly even if you went for an 8kg aero bike (you'd be able to get a fair bit lighter 100%) you'd still feel a decent and noticeable benefit over your current bike (stiffness in the right areas may also help give the illusion or reality of higher speed).

In all honestly I'm not sure you'd save more than half a kg these days anyway if that on a lightweight bike vs aero? 

Personally I do feel aero gives a benefit. I have a canyon ultimate with 60mm carbon wheels and the wheels certainly made a difference compared to the normal ones. The frame will make slightly less difference but a number of riders have told me their canyon aeroad is good for an extra 1 mph over their previous lightweight bike. 

Whatever you get with your budget it should be a big upgrade and all I can say is enjoy the looking/test riding!!

Aero will probably get you the best 'speed' gains..It'll be more beneficial the faster you go, you should believe the hype, however you don't need to by an aero bike to make aero gains, the bike is only a small part of the aero puzzle. 

Position on the bike should be the first place to look,  baggy clothing, helmet, deeper section wheels.. 

Of course none of the above will change the fact that stronger legs/heart/core/flexibility are really what it's all about. Depends on how serious about it you want to get. 

Still enough about that.. you're buying a new bike! Go and demo some! I'm a fan of Giant bikes because you get a lot of bang for your buck and I have a local dealer/shop who I can go to. Take a trip to your local bike shop and see what they have, all brands have great bikes at your budget you're spoiled for choice!

Canyon Ultimate CF SL 8.0 Aero- ish £2700