that's an awesome idea! However, i must warn you about shin splints. If you don't know, shin splints are when the muscles in you leg (not sure which one) begin detaching from your bone (shin). If you really do have this, you need to take a break from running too =(. I used to do x-country and track, and i saw many people get shin splints and not be able to finish the season.
As an alternative, i would suggest walking or hiking. Walking in a fast pace will actually be better for you (your knees mainly) and will also help build muscle. I walked home 2 miles (i know, its not a lot) in a VERY fast pace for a year and my mile-time dropped form 8min. to 6min. Just by doing that little bit everyday i was able to gain so much time, imagine what you can do with a full workout every day. I love running too, but i just want you to be careful, and shin splints are something to be careful with. Also, remember to be careful running steeply downhill... it's the easiest way to KILL you knees. And you need them for riding ;)
I hope you can keep running, and good luck to your horse as well!
oh yeah, you can also wrap for shin splints, but then you need to ask some professional because i have no idea how to explain it without showing you!