Thanks everyone,

In answer to your questions: I'm 29 and whilst not exactly a couch potato (I walk a fair bit and go to the gym a couple of times a week) I'm definitely lacking in the fitness stakes!

On the MS bike ride website, it classes the ride as "easy" so I'm assuming that it will be smooth, non-hilly-ish terrain. The local ride is in a couple of weeks time (definitely unrealistic!) so I'd be entering the one a couple of states across which is in September. Unfortunately I don't think I'd be able to find a cycling training group to join locally for that ride.

Thanks very much for your advice - which has made me think it's not completely unrealistic! As a new cycler, I'm a little worried that by taking this on it might take some of the enjoyment out of my new sport if I'm riding to discomfort. So I need to weigh this up in my mind.

Thanks everyone - really useful advice - much appreciated!