Anyway, I consider Bustitution to be a necessary evil, so long as it happens to someone else. However when National Rail start recommending allowing an entire hour to get from Cambridge to Ely, and you live in Girton (a good 20-30 mins from Cambridge Station) you start thinking "I could do better".
All would have gone well, except that I couldn't find the map with Girton and Cottenham on it, but I did consult other maps and decided that that bit was easy. Unfortunately conditions actually in Cottenham disagreed; particularly in the way that the signpost (pointing to Rampton one way and Histon the other; neither of which I wanted to go to) was unhelpful. Still, I soldiered on (wrong move!) and got to Willingham before deciding I was definitely on the wrong track and phoning ahead for directions.
Blue shows my planned route (and the route I took home), Pink is the diversion I actually took.
Still I eventually got to Ely where, having just missed a train, I ate lunch in the pub by the river and got the next one. A lovely afternoon with my parents and grandmother ensued and then I returned, with none of the mishaps that catagorised my outwards journey. The return route has a couple of very nice down-hill sections, and runs past an "Old Station" that looks nothing like (and four Station-worker's cottages, which do), along what must be a road on the old railway route.
My estimate is about 45 miles for the entire round-trip cycling, over 4.5 hours (2.5 out, 2 return (more leisurely)).
ETA: Google Maps makes it closer to 42 miles.