PG3 - I am well into sauna training, but the reasons are several:
1.) I have Two Oceans in about 6 weeks, where the heat can be severe at times
2.) I am very heat intolerant in general
3.) While it only takes 2-3 weeks (some say 1-2 weeks) to achieve heat acclimatization physiologically (increased plasma volume, increased and earlier sweating), it takes longer to achieve psychological acclimatization
4.) I want to be able to be in the sauna for over an hour at a time, and it takes a mont to so to work up to that safely (I am at 50 minutes now)
I'm doing strength training this time around, and the exercises that Coach Parry suggests. I have also added sauna acclimatization - the research suggests just spending time in the heat will do it, you don't need to exercise. I'm doing that 4-5 times a week (if I had hot yoga close by I'd do it).
As for downhill running, I have a treadmill that goes to a 6% decline, and it downloads routes from the internet and emulates the elevation changes - so I run 8-14 miles of the Comrades route that includes some big downhills.
You'd think with this preparation I'd be better than I am!