I don't mean to kick a man while he's down, but I think its pretty likely it was over-use. Plantar fasciitis is a classic injury of that type - a slow and gradual over-use leads to increasing inflammation and tightness. Eventually you reach a tipping point and it gets very sore - in your case the additional stress of finishing a long run with a higher pace.
As someone who's had a lot of running injuries, I recognise this pattern. You've ramped up from not much at all to a pretty hefty mileage of 40 odd a week with no rest weeks and 33 days in a row - that's a lot of training load, even for people with years of consistent running in the bank.
Once its settled you'll need to get back into it slowly; start with low mileage and don't add more that 10% a week to your weekly time on your feet of the distance of your longest run. Have rest days and plan the training as slow and progressive overload.
Running is the most unforgiving sport for not managing training load carefully - I've got 2 hip tears, a cruciate ligament and medial ligament that can attest to that!
Spot on, sadly. Any decent coach worth his salt will tell you rest days are as important as hard training days...
Anyway, you're already on the recovery path, hopefully won't be too long.