Bristol to Reading

Can the above be done in 20 hours and back

Yeah, if you're fit. I did Oxford to Weston-super-mare in about 12 hours, at a leisurely pace, quite a few lunch stops etc.

There and back's gonna be pretty unpleasant though.

it works out at 100 miles each way, if you average at 10 mph. maybe it could be done

There's a Sustrans route all the way if you want a quiet route
http://www.sustrans.org.uk/sustrans-near-you/south-west/easy-rides-in-the-south-west/kennet-and-avon-cycle-route

It reckons 10.5 hours Bath-Reading, assuming you're reasonably fit you could probably do Bristol-Reading in 10, depending on how many walkers are in the way.

Are you going by road or the Sustrans route? The road is much quicker – I did it in about 5 1/2 hours last year. Sustrans route (NCN4) is much more pleasant than the A4 though.

i ride the sustrans route fairly regularly – there are lots of walkers, fishermen, joggers and gates all of which prevent you from getting a serious move on. unless you are thinking of doing it in the evening [when you'll be plagued by flies and midges] 12-14mph average is acheivable otherwise it's smooth and fast so 15mph+ average is a reasonable pace even with the gates you'll have to go through. i live 10 mins from this route and mrs mattp makes nice cake if you need some sustenance 30mins from reading…

poss, we had real issue as there was an angling match on and quite a lot of twunt anglers resented having to move poles (ie cusing us to stop), even though they were blocking a ROW
plan a route near railway then you have a plan b