Lots of stretching, before and after exercise. I learned this the hard way when I developed plantar fasciitis last year. It's usually a direct result of a tight achilles tendon and certainly was in my case.

If you really want to get sore, try hiking at the beach. It's amazing how many otherwise unused muscles come into play when walking on sand!

Just plain walking for 1/2 to 1 hour, 3 days per week, plus one longer hike (2 hours or more) on weekends, plus the strength training you're doing, should help considerably. If there are hills near your home to walk on, so much the better.