Riding actually doesn't bother my back very much, so long as I stay in the saddle and keep my core active. It's the twisting and rotating (and use of my lats) that really kills me in the pool - I'm going to give myself another day or two before I (gingerly) try to get back into the water.
Anyway, I managed an 88-miler with some pretty intense segments in the first 40 miles. Good weather, favorable winds, good stuff. Then I had the shop take another quick look at the placement of my cleats before swapping my bike shoes for sneakers and logging a 4.5 mile run.
Running actually does hurt my back a little bit - my upper body definitely isn't as stable on foot as it is in the saddle. I might take it easy there for the next few days, too... but I've gotta do what it takes to be ready for the half-marathon I'm signed up to run next Sunday.