Glenfield east to Sicome is very flat with only a couple rolling hills. For no hills at all park at Sicome in the back entrance (closest to 22X) and you can go for about 5k's along the river on new paths that aren't damaged like some of the paths near glenfield. Stay away from the West side of the park by Marshall Springs and the biggest hill in the park at 37 Street

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Depending on the distance you are looking to go for you may want to park by 52 Street NE and there is a path that goes out to Heather Glen if you go east or you can go west to Deerfoot and either way is a good by east has less other people on the path so likely better. Very smooth paths.
Another great spot but can be busy is south glenmore park to weaslehead.