Plantar Fasciitis is actually a fairly simple thing to fix if you're on top of things early. It's very possible you were wearing sprinting spikes that had a negative heel-to-toe drop and lengthened the fascia more than you were ready for.
For me just changing shoes with a 6mm difference in heel-to-toe drop was enough to create the same problem. I woke up one morning with a pain that felt like a nail being driven up into my heel. I spent about an hour doing deep tissue massage on my arch and calves - did a workout the same day and didn't have any issues the next morning.
You probably don't need quite as much support as you might think if you were willing to slowly remove support which would strengthen/stretch the muscles around the heel.
You may have just misspoken but it'd be almost impossible to get any anaerobic training out of downhill running. You might be doing some muscle strengthening though.
Running on an uneven running surface isn't bad for you at all. Most people run primarily on the roads - so if they encounter a long path with uneven terrain - the ancillary muscles that control stabilization generally tire more quickly. It's all about transitioning slowly and letting the muscles, that haven't been used very often, strengthen.
For me just changing shoes with a 6mm difference in heel-to-toe drop was enough to create the same problem. I woke up one morning with a pain that felt like a nail being driven up into my heel. I spent about an hour doing deep tissue massage on my arch and calves - did a workout the same day and didn't have any issues the next morning.
You probably don't need quite as much support as you might think if you were willing to slowly remove support which would strengthen/stretch the muscles around the heel.