I am a runner and walker, and one thing I was pleased with at Riviera was the great running abilities around the lake and through Caribbean Beach. I imagine putting together a 5 k run in a couple of laps would be quite easy, adding in a detour through CBR.
Alternatively go to Epcot- takes about 10 mins max on Skyliner but may not be running early enough, not sure. If you run three times around the lake there, with 1 detour up to Hollywood Studios and back, it’s about 5k. Each circuit of Crescent Lake there is 1.2k I measured it on my GPS watch.
Riviera gym also looked OK, but the usual lack of running machines which all resorts seem to suffer from and in bad weather they often all get taken. Luckily this will not be an Animal Kingdom situation where they are taken all the time due to lack of outdoor running area. My wife and I do not like AKL for this reason. As above, desperate runners are seen running up and down the car park there.