I’d be bored sh!tless doing 50miles of Swinley.
As for the mud though, suck it up, it’s winter. Bikes don’t melt in the mud. 😛
I find the cobbles are harder work than slogging the odd bit of fireroad round there. But get off piste and over Crowthorne way and you’ll get a better ride, and yes, extend it to TH etc for the miles and way more fun. There will still be mud though, just not ginger 😉
A decent 50 miler though off road… Surrey Hills, combining them all. Big killer loop (and I mean killer). Winterfold, Pitch, Holmbury, Leith, Box, maybe Mickleham and Ranmore, taking in as many trails as possible. Should do it. 😀 . Most I’ve managed is 45, and took all day.