Any rides for beginners?

wvm is right: if you want to do an out and back from home you will not get much countryside for 25 miles. If the ride out of London seems like a bit of a grind, don't rule out using Oyster cards for quick trains which can pop you outside the M25 for ±£3. It's well worth doing, many of the TNRC routes are good for that, the Hemel Hempstead one may be closest to you.

The lea/lee valley canal path is good, but it isn't paved, so you'll be dodging walkers and families etc. That's probably about 25 miles to the end in Ware/Hertford, and is lovely in spring summer. I've also ridden out of London on the Paddington canal path to Iver, which I enjoyed.

Otherwise, there are tried and trusted routes to Cambridge, Southend, Brighton which you could do partially. And then inevitably steam onwards when you realise how much you are enjoying it....

Thanks for the tips. Can I just turn up unannounced at Marylebone and take my bike on train to Amersham or somewhere? Or does one need to book a bike space?

Also great tip about the canal at Regent's Canal, that's right next to me but for some reason it hadn't occurred to me to go out along that.