Easy cycling - what coaches call 'active recovery' is great, especially if the resistance is low and cadence (rpm) is high. Walking also excellent, and one day if you go for a brisk walk on a non-running day you might even find yourslef jogging a few paces now and again!!!
Have you thought about doing upper body strength work and core work on your non-running days? Even a few sets of press-ups (one of the single best exercises) wil help, pull-ups although a lot more challenging also excellent.