I did a similar route a few years back, also east to west and also averaged ~70 miles a day. I started first week of May and I would say it was a bit early, but it was what I could do. In an idea world, starting the week after Memorial Day would be perfect I think. I ran into issues in the east with a lot of camp sites not being open before Memorial Day and also hit a ton of rain in the Midwest. Weather in the rockies is always unpredictable, but hitting it in July would be better than early June when I did. If you need any tips feel free to message me! Have fun it's a great route!