Running is my primary sport. I'll usually put in 85-100 miles per week. 5-6 days of doubles, with an easy short run in the morning and my main run later on in the day. Two days of speed work (one track, one tempo) and one long run per week. Other days are either medium effort or recovery runs. Example track workout might be 1600m at 10K pace, 400m recovery jog, and then 12x400m at 5K pace with 100m recovery jog between repeats. Tempo work is generally 3-5 miles at faster than lactate threshold pace, or cruise intervals (e.g. 3x3200 with 5 minutes recovery between repeats). If I'm doing something else (mountain biking, road biking, etc), it will go on a recovery day and I'll still get a run in that day (distance will depend on how hard the other workout was). I do the majority of my running on roads, with some trails or fire roads when I get a chance. Track intervals are done on a track...other than track workouts, I stay away from that godforsaken thing. It might be soft, but running lots of tight loops is a good way to wreak havoc on your inside leg (left knee and IT band usually). Also, my workouts are set up as half marathon training, with the plan to eventually move up to the marathon, so they're a little on the long and slow side for 5K's. When I was 5K training (aka when I still had fast twitch muscles left), I was doing shorter, faster stuff (e.g. 10x400 at 3K pace), and was doing a little less mileage.

I guess it helps with long climbs and long rides, but I firmly believe the best cross training for a sport is the sport itself (i.e. the best cross training for mountain biking is mountain biking). Might get you in shape for some Xterra races though.

Those Adidas bouncy shoes look like a gimmick to me. I don't think I've ever seen a serious runner wear something like that. The "running" shoes you see in stores like Foot Locker are often no better than the "mountain" bikes you see in Walmart. A "mountain" bike is fine for riding to the store, and "running" shoes are fine for walking around the block. But even if you're only running a couple of miles per day, it's still actual running and your feet and legs will be best off with a pair of real running shoes...it's no different than saying you're only going to only hit the small set of jumps with that Next bike. Any specialty running store can set you up with a good pair of shoes though...they'll put you on a treadmill and analyze your gait. What kind of shoes you need will depend on your biomechanics. Your foot pronation will determine how much stability and cushioning you need in your shoes. And more stability/more cushioning isn't always a good thing...I wear a neutral shoe with no stability features since I have neutral biomechanics. An overpronator with flat, flexible feet will want a motion control shoe, while a supinator with high arches and rigid feet will want a cushioned shoe with no stability features. Wearing the wrong shoes for your feet often leads to injury and could be the cause of your shin pain...and this holds true whether you're running 20 or 120 miles per week.

Also, shin splints is a catch-all term for shin pain. By your description of it not being "shin splints" though, I'm assuming you've ruled out medial tibial stress syndrome, which usually strikes beginning runners and is caused by increasing speed or distance before your body is ready. Other possibilities are compartment syndrome or a stress fracture, but before assuming the doomsday scenario, I would suggest stretching your calves before running, icing your shins after running, and running on a softer surface if possible (dirt roads and grass are the best, roads are better than sidewalks, and if you can stand them, treadmills are usually pretty soft).