What i have done is alternate between a challenging route, and a less challenging route, listing ot my body, if it needs a break i take it easy, if not i tend to push myself very hard. For example, i can get to a point 1.5 miles away in 18 minutes or 7 minutes on the same route. I just have to decide where on that scale i want to land. I know what spots are harder for me, and which parts are easier, so if i really want to push myself, i have to know when to ease up so the hard parts do not burn me out. You could apply this to your routes and build up endurance. Keep in mind that it will take time no matter what method you use, be it this one or simply longer or more challenging routes, so don't get frustrated or despair.

- Andy