That's what I did today at the gym. The run was 5K with a medium/hard tempo. By the time I got downstairs where the spin room is, the class had already started. I changed in to cycling shorts, filled my water bottle, put on my SPD-compatible Pearl Izumi shoes, and then got on the bike a few minutes into the class. It ended up being very, very difficult. Definitely makes the top 5 in terms of difficulty. Lots of climbs and sprints. During one sprint my right calf 'locked-up' and I had to get off the bike for a couple of minutes to stretch it out. It was better later on and should be OK by tomorrow or the next day. Overall, an awesome workout! Maybe I can add swimming to the repertoire in the future (no pool at the gym though)?
Total:
5k Run (treadmill)
+ 15mi Bike Ride (spin class)