Coast to Coast Pyrenees

Any sense upgrading parts on the bike? I'd say no unless they are proving unreliable, uncomfortable or inadequate. This is my own personal view born of experience rather than expertise - doubtless others will disagree!

Inadequate:

In your case, I'd follow CXR94Di2's advice and get a rear cassette with 34 teeth on it. I think for your weight, a 50/34 front to 12/28 back isn't going to suffice. The 34 front/28 back gear isn't going to be low enough for all the climbing. At some 3 stone lighter and 5 years younger than yourself, plus having been cycling for much longer, I max out on climbs which reach 10% gradients or so with that gear. I figure you'll encounter quite a few bits of the route like that, day after day. 

Watch out though - for such a large rear cassette, you'll need a "long cage" derailleur - the "GS" model as oposed to the "SS" model. It has a longer arm between the two jockey wheels to accommodate the large cassette and which takes up the extra slack in the chain when on the small rear ring.

To see what I mean, Wiggle have it here: http://www.wiggle.co.uk/shimano-tiagra-rd4700-rear-derailleur-gs/

They've also got a Tiagra 4700 11-34 rear cassette here: http://www.wiggle.co.uk/shimano-tiagra-4700-10-speed-cassette-11-3211-34

If you do go for the large cassette option, you might need to get a new chain with a couple more links if your existing chain has been trimmed down. I'm not well enough informed on the optimums for chains though. There are 114 and 116 link versions - I suspect you'd need the 116, but check with a qualified mechanic.

One other area to check out - wheels are probably fine, but tyres supplied with new bikes are often not very good. You'll want something that is comfortable, grippy and puncture resistant. There's as many opinions on the best tyre as there are cyclists - the forums will demonstrate this. For my money, Continental GP 4000s ii tyres have served me very well on those fronts. Consider a 28mm width tyre (if it fits within your frame) to provide a softer, more comfortable ride - you can run them at lower pressures that the narrower 23mm and 25mm equivalents.

Uncomfortable:

You'll be on the bike day after day, and in the saddle for several hours in each. Make sure that you are in a position you can stay relaxed in. This may mean getting a longer/shorter stem, or turning the stem upside down to raise/lower the handlebars. If you and the saddle don't reach an accommodation, consider a different shape saddle or a seatpost with offset. Bike fit is very personal - find something now that works for you and do not change it until you've finished the event!

Unreliable:

Again, you're on the bike for a long time. If something is clicking, creaking, squeaking, rubbing or rattling and you're anything like me, it will drive you mad very quickly. In the worst scenario, such noises may also be a sign of something failing. Make sure that all the bolts are properly tightened to their specified torque settings. In my book, a bike shouldn't make any noise at all. 

Check your brake blocks for wear and/or embedded debris. Clean the braking surfaces on your wheels. The descents will be steep and frequently have hairpin bends along the way. Your brakes have to be working properly.

Lastly, and quite importantly in my book, make sure that you have got your bike the way you want it at least 3 weeks in advance. Don't put any new bits on after that unless you're going to be able to give them a really good shakedown. This goes for inner tubes and tyres - you really don't want a pinch-flat if you hit a stone going down a steep descent.

It all boils down to having confidence that your bike is in good shape and can be relied upon, leaving you free to enjoy the cycling.

I hope that's of use. It isn't to be taken as gospel - it's just what's worked for me. If I've got anything wrong, I'm sure folk here will set me right.

I really hope you enjoy the event.

European mountain hills can be 4 to 15 miles of climbing. When you have a clear day, try riding up your local steep hill and then do it again and again and again. Pace your climb, you could destroy yourself on one hill if you try too hard, don't chase the small riders. If you completely die trying hill repeats then easier gearing will be a necessity.

A mate of mine did the Wiggle punisher 100 miler in his first year of riding a road bike. I fitted a 11-34 cassette to his bike. He said it helped, but also said it was the hardest thing he ever did whilst on a bike. He was totally wrecked. He was 16 stone like myself.

Looking at the c2c pyrenees classic route there are some big climbs. They mention 4 hrs per day riding but some of the days are upto 100 miles. How many days are you setting aside to do this route?

Given you are a big guy, physics is against you and you will find the hills particular hard. If you can fit an easier cassette something like this
http://www.wiggle.co.uk/shimano-tiagra-4700-10-speed-cassette-11-3211-34/
Get the 34 tooth version. This will help a little, but expect it still to be hard.

Ideally you would want to have mtb crankset(front gears) something likehttp://www.wiggle.co.uk/shimano-xt-m785-10-speed-hollowtech-ii-double-ch... 38/24 setup but if you're in-experienced with bike mechanics then it would be best to let a shop do the work. A shop can prepare all the bike for adventure.

Re training. Go out and find some similar or steeper gradients and see how you cope with climbing them repeatedly. If you cope ok simulating a pyrenees mountain route then maybe you will be okay.

You have chosen a difficult multi day sportive for your first outing but good luck in June