Let your body be your guide. What works best for you, may not be what works for others.

It is good to take a couple of days off each week when first starting. A 20 mile ride (half your round trip) is enough to start building strength and endurance.

You might want to try and split your rest days. Ride half your work commute on Monday, Tuesday, Thursday and Friday. Taking Wednesday off and Sunday off. On Saturday, start off trying a 30 mile ride and see how it feels. If it feels too easy, do 35 or 40 next Saturday. The advantage of the Saturday ride will be that you will build you endurance faster and by splitting the recovery days, you can build strength faster.

After you build your Saturday ride to 50-60 miles, then it would be a good time to try a round trip commute on maybe Tuesday and drop the Saturday ride back down to 40 miles. Until your body catches up to two 40 mile days in one week. Keep working up along these lines.

If the above plan would not work for your schedule, then figure one out along the same idea that fits. Just be sure to keep it fun.