How about using bikes as return transport;
Drive to anticipated finish point for day 1's run with the bike in the back of the car and lock them up there, then drive back to the run start.
Run the 15- 20 miles and cycle back to the car at the start, then drive on to Day 2's finish location and lock the bikes up again and drive off to your accommodation.
Following day drive to day 1's finish and start day 2's run.
It is a bit of a palaver and does add 2hrs extra exercise to each day,
(or you could do the same thing with two cars, quicker, less effort)