Honestly, I found that at my level of play I was sufficiently fit to handle any 3set match simply by having one non-stop hour and a half long hitting session per week.
Just playing 3 games of 21 with little rest you will hit way more balls than you will in a 3 set match and be far more gassed.
I'm not sure how that applies to 5.0 and higher players, but watch a WTA match even. Some of the top players aren't exactly in great shape. Kvitova has love handles, Serena ain't sleek. I think long distance running for tennis is way overrated.
3.5 player. Equipment: Prince NXG OS, hyrid kevlar mains/poly crosses 50 lbs