When running is off the table, cross-training can keep aerobic fitness going. Which option works best depends on the injury and on what you can do without pain.
Deep water running (aqua jogging) mimics the running movement with no impact; it has a long history in rehab settings
Cycling, including indoor trainers, allows high intensity with low impact, though heart rate is often a little lower than in running at the same effort
The elliptical and cross-country skiing or ski ergometers are other low-impact choices
Doing your usual hard sessions in the cross-training mode helps maintain fitness better than easy work alone
A pool session that mirrors a running workout: 10 minutes easy aqua jogging, then 8×3 minutes hard with 1 minute easy, then 5 minutes easy. Use perceived effort rather than heart rate, since heart rate in water usually reads lower for the same effort.