Running break

After a long run (of 13 miles), I would take the next day completely off (full rest/recover day). By the second day, I would do an "active recovery day" of walking or light biking. By day 3, I'd start light running again (maybe around 3 miles or so at an easy pace).

The best way to prevent knee pain is to take rest days as needed, change up your running surfaces/paces, cross train, and strength train. Strength training once a week for legs should be fine (quads, hamstrings, and calves). Squats, lunges, leg curl, deadlifts, and leg extensions are best.

For myself, if my goal is improving my time for my next one, one day off then run a 15K. Strengthing the knees... not sure why you would want to strengthen these - if they are sore or something like that - maybe it's not strengthening that you need, but work a complimentary low impact exercise instead of a running day - like biking or swimming.

How long you wait depends on your experience and how hard you ran your half marathon.

A half marathon doesn't beat you up quite the same way as a marathon, but if you do it at the best pace you can then it'll still take it out of you... so by all means take a day or two off running afterwards.

But as long as you don't have some specific injury then there's nothing wrong with having a gentle run the day after a HM, but keep the pace and distance relatively low (especially the pace). People focus on distance without thinking about pace; but the effort involved in running is very much a function of both... of course different people have very different paces that they're capable of; so you have to learn for yourself what counts as an easy pace and what counts as hard.

If you run regularly it's a good idea to follow the "hard, easy" principal... every time you have a particularly hard running day - either by way of pace or distance - then have an easy day to follow; or a rest day.

If you take rest days from running and want to keep exercising then do some cross-training; anything that gets your heart rate up for a significant period of time is fine. Cycling, rowing, swimming, cross-country skiing, hiking...

I will wait a day or two then do a short (4-5mile) run.

Liar! You ran 5 miles the day after your last 1/2 marathon pacing me on my long run! (thank you)