Lake Elkhorn is probably your best bet if you're looking for decent length and flat. It's only got one real hill on it, and if you go around the right direction it should be easy enough to avoid either climbing too steep or gaining too much speed.

Alternatively, the trails behind Wilde Lake are pretty flat, but tend to have some debris on them that might trip you up.

There is also a trail that connects almost all of those parks together, and from what I've seen through google maps its pretty flat and clear. I think it starts somewhere in Savage and goes at least as far north as Centennial.