One way is about 56 miles (according to google cycle route planner). I’ve ridden between Reading and Bedwyn lots, and the surface varies from fairly narrow and bumpy gravel or dirt, to well kept towpath. The variation keeps it more interesting and it’s all fine in the dry. It can get puddly, gritty and muddy after a wet spell though. Going from Devices (top of Caen hill flight of course!) it is at the top of the canal so it’s all gently downhill, but of course it is just flat sections with a small blip up or down for locks, depending which direction you’re going.
If you’re not so fit then a 112 mile out and back will be fairly tough and a bit boring covering the reverse of what you’ve just done. Maybe try a return trip to Hungerford or Newbury instead, it’s lovely countryside the whole way and some nice pub/cafe opportunities to be had. If you need to bail or shorten the ride you can always catch the train to Bedwyn or Pewsey and cycle the rest home. For that length of time on the towpath, be sure to take a bell!