* Running Rewired, by Jay Dicharry: https://www.amazon.com/Running-Rewired-Reinvent-Stability-St...
* Anatomy for Runners, by Jay Dicharry: https://www.amazon.com/Anatomy-Runners-Unlocking-Potential-P...
* Run for your life, by Mark Cucuzzella: https://www.amazon.com/Run-Your-Life-Without-Well-Being/dp/1...
* Running Science, by Owen Anderson: https://www.amazon.com/Running-Science-Sport-Owen-Anderson/d...
The podcast "Science of Ultra" is also great and strongly rooted in a scientific approach to claims about running and training.
I started to feel some tightness in my left hip after running longer distances (25+ km), and went to PT. I basically now have a strong program of cross strength training that closely mimics the exercises in the Dicharry books, and I am know using muscles that were neglected before (glutes, mostly). I haven't felt any pain / tightness since.
While I don't have much of a background in sports, I do play bass and piano, and of course sit at a keyboard all day, which are also highly "injurious" activities. If something hurts or feels wrong, it is wrong, and you should address it. This is absolutely true for playing and typing, which should feel effortless at all times. I don't have enough experience with running to know how well that applies, since adaptation and training stimulus doesn't feel the most comfortable.
I can't recommend this book enough to help identify (and address) weak points.
First of all, kudos to you for zeroing in on two crucial form issues! I wish I'd done that at your age, instead of spending years developing bad habits that I had to unlearn!
Jay Dicharry talks about these exact two issues in Running Rewired. This is the best $20 you'll spend on a training book ever.
Your calf tightness could be a couple things, either needing to be loosened up with rolling (you can try a regular foam roller, a harder, knobby one called a Rumble Roller [what I use], or a lacrosse ball for pinpointing spots). Or, it may be your joints are too stiff, and you either need massage or stretching to increase range of motion. (He has diagnostics for these in the book).
Tight hip flexors were also a problem he diagnosed me with (very common with runners and people who sit down a lot). It's very simple, but not a quick solution: do a hip flexor stretch for 3 min per side, daily, for 10 weeks. That'll allow you to get your leg behind you more, which makes all the difference in efficiency, especially (as you've noticed) at faster speeds.
As you're already guessing, the other side of the coin is glute activation (another problem I had). There are lots of great resources on youtube for glute activation exercises, but a few that I've done hundreds or thousands of reps of are clamshells, glute bridges, and donkey kicks. (Note: concentrate on your form! Be patient with yourself, and even put a hand behind you to make sure you're squeezing that glute, and not cheating with your lower back and hamstrings, because that defeats the purpose.) Building new muscle memory takes time, but if you stick with it, you'll see major gains and be able to feel it in your stride. I sure have! I'm getting injured less and running faster than I was 5 years ago. :-)
P.S. for a quick intro to JD's work, check out this podcast: http://runningrogue.libsyn.com/episode-80-rewire-your-running-with-jay-dicharry