Water running can help runners improve running form. The resistance of the water forces the legs to find a more efficient path. Put on a floatation device and go to the deep end of a swimming pool (usually at least 5.5 feet deep). Move your legs in a running motion that replicates how you run on land—no knee lift, extending your leg a little forward, feet low but kick out a little in front, and bring the whole leg behind as the lower leg moves behind you. Start with 5-10 minutes and build up to 15-30 minutes. You can do this workout on a non running day, or as a second workout on a running day.
I have not tried this, but a number of my running friends regularly do this in the pool at our health club and they have been pleased with the results. It seems to me this activity is particularly useful to the 40-plus folks who may be experiencing some fatigue or soreness.
Thanks for the tip!