85km long race doable for a new cyclist?

Bobbinogs.. +1 yes..
One of our local events is Coast to Coast. About 100km. Initial climb to a roller coaster gradual descent. I ain't as fit as you look in your picture bro.
Local coppers and I basically made a group. Chatted away many hours.. Blah blah blah. I was on a smooth light weight. They were on mountain bikes. Made about the same speed.. About 3/4 race distance they said goodbye.. I queried if it was too far.. They weren't even tired.. They ride every day all day and their shift was over, dog box about to pick 'em up, paid to ride..
You'll be alright.. Bigger legs than mine bro..

Your club riding friends will have a much greater appreciation of your ability than this forum. Have you spoken with them yet?

If it's at all hilly, for an initial event, I'd train for the ascent first, distance second.

OP: have you got a link to the actual event in question?

Plus a bit  more about you - what bike you've got, have you ever done any group riding, where you're used to riding (ie is it hilly, flat, urban...)?

Yepp, the OP needs to be a bit clearer on what they mean by "Race".  Most new/non cyclists refer to any organised event as a race, whereas the majority of events are just that, an event to be enjoyed at a comfortable pace.  OP, if you can 25km now, then you should be able to do an 85km event in a month's time.  Just ride regularly (more than once a week) and aim to increase the longest ride each week by a reasonable amount (say 35km this week, 45km next week, 55km the week after and then 85km on the day).  Keep your pace steady on the day and don't get swept up into the thrill of it all by working too hard for the first half.

If the OP really does mean "Race", then I would say probably no chance, but that is very dependent on natural ability and the level of racing.

Are you going to watch a race or competing ?

Cycling 85k would be doable with training.

Racing 85k is a different matter altogether.

Are you going to watch a race or competing ?

Cycling 85k would be doable with training.

Racing 85k is a different matter altogether.

If you're the guy in the picture then a sportive over 'normal' terrain (=no mountains or big hills all the time) shouldn't be a problem provided your equipment is ok and you start building up the distances you ride over the next weeks.  Even if you don't prepare but are in good shape then yes, you'll be able to do it but whether you'll enjoy the experience or be able to walk easily the next day are different matters.

If it's a race with trained participants then the short answer is likely no, except if you have the condition of a top athlete.  Speeds are usually high and you need to be able to keep them up over time.

Read up a bit on (competitive) cycling:  it's about more than just showing up with a nice bike and in decent shape.

85km race? Or sportive? I wouldn’t expect you’d enjoy trying to hold race pace for that long, is that you in your avatar?

Depends on your level of fitness. How did you feel after 25k?