How often do you plan to commute. Daily? Then be prepared for an adjustment period. You be may be fit, because you run a lot, but still 40 miles a day is loooong. the first few weeks you will be tired once you get home. Eat enough, get enough sleep. After 6 weeks or so you will start to feel better about it

You could consider driving to work one day and taking all your clothes and food for the week. That will make your ride a lot easier. If that is not an option, then invest in a good rack and trunk or rack and panniers to carry your stuff. 20 miles is long way to carry stuff in a backpack

Also, know your own limits. There is no shame in driving to work if the weather is too cold or there is snow and or ice on the road, no matter what some of the hardcore BF'ers say
Duppie