Considering only net force when walking or running is misleading. Otherwise running with a constant velocity in a flat lane would be as effortless as biking, because the air resistance is small at that speeds.
If we run completely upright, at each step the resultant force direction between foot and soil can't be horizontal. If it were, there would be a torque and we would fall backwards.
That means there is a vertical component besides the horizontal one. We run making jumps and it is that work against gravity that makes us tired.
But we can run tilted forward, so the vertical component is smaller. The torque due to the horizontal friction force is balanced by our body inclination. But even so, there is always a vertical component, and we work against gravity.
Of course, there is an extra effort when accelerating, but most of the effort is related to keep a bigger velocity (higher or more frequent jumps) instead of small one.