Personally as someone who use to do a team sport and run, then in the off season do resistance training, run and race, I would do 3 running sessions and two weights sessions a week. Then once I increased the running I would try and still do two resistance sessions a week even if one of them was after a shorter easier run.
Why? If your long term aim is to be a better runner than running is what you should do more of. It will ensure that the weight work you are doing is complementing your running rather than hampering it. If you continue the resistance/weights while running then you won't lose any of the strength you have built up particularly in your upper body.
The advantage of training for most other sports is that they all include an element of fitness/circuit training in at least one of the sessions so you get to do sprint work and resistance training in one go.
As you don't want to do this I would aim to do 2 of the harder sessions a week plus the long run. Then if you are tired in a week (which you will be) do one of the harder sessions e.g. fartlek, speedwork, an easy short run and a long run. However the long run can just be for distance that way when you start your marathon training you don't have to start building up distance from a low base.
There are certain days and certain runs when the endorphins flow.
It took me about 4 years of running before I started getting them. One day I went for a run in the local Royal Park on a horrid wet Autumn day, and I just got a rush.
Running before then wasn't a struggle as I start running to keep fit for other sports. I was use to going for the gym and doing exercise classes before then so I was toned and had some level of fittness but doing team sports meant I had to build my endurance up.
Now I suffer from a health issue that makes running unpleasant and lots of others sports off-limits, but when I can run I can still get the endorphin rush.
I suggest you go and get it properly diagnosised by a sports physio asap.If you in a running club get one recommended by someone there, else state your location and ask on this forum.
BTW my plantar fasciitis pain started in the arches of my feet not in the heel like other people and I didn't have problems on waking unless I had pain the day before. I also suffered from ankle, knee and hip pain until I sorted it out. However I do have old ankle injuries.