Since I hit 50 (late 40s, actually) I am finding that I cannot comfortably run back-to-back days. I currently run 4 days/week, and that is probably pushing the limit of what my knees want to deal with. I am running about 25 miles/week, and I can ramp up to ~35, but I cannot get much more without persistent soreness and increased injury risk. So, adjust your # weekly runs and distances according to what your body can handle.

Biking, on the other hand, is easy. I double-up bike and run days with very little impact to either workout. So, give it a try, but you may find that you can ride on a run day without impacts. I ride 2x/week on run days and 1x/week on swim days.