Couch to 40km charity ride?

If you push it on a flat course, like 85-90 mins, if you take your time and enjoy, maybe 2.5 hrs. It's really going to depend on your bike and the course too.

Mostly flat, few small hills. I have a Felt S22 which like i said i have only been riding on the trainer.

Is there anything i can do (as a noob) to help with my prep for the ride?

With only a couple of weeks I would just ride as much as possible, take a couple days off before the actual race and make sure you go into it fully hydrated. This needs to start at least 48 hours ahead of time.  Just relax and have fun.

Felt S22 ? like this one ? : 

clicky

just asking though.. being a charity ride, you will probably be riding in some sort of bunch.. you probably need to be training with bunch riders, because if you come in green as grass it might not be a nice experience.

also.. you probably wont be going aero too much, and riding on the horns is not that nice for any length of time. I have actually changed my aero base bar to a carbon drop handlebar and put brakes with hoods on it.. might seem like a backward step, but I bought the bike for the geometry.

its a charity ride, not a race....go out and enjoy it.

Dont worry about not finishing it, there will be young kids on BMX bikes that will finish. Have fun, enjoy and dont worry about it.

Get out and ride. 

Yep thats my bike - have been thinking about the drop handlebars. Will give this ride a go and see how it feels. Am going with a friend and i will try and hang at the back thats for sure. I will only be going slow for my first ride outdoors

Looking forward to it!