How to get going again after injuries and stagnation?

I'm hoping that the collective wisdom of this community might be able to help me with a running reboot, after some pretty bad years.

I'm at a bit of a loss trying to work out how I return to training properly in a sustainable way, and I'm not really making much progress by myself.

I'm struggling because I'm not exactly a novice runner but it's been so long since I was running at peak form. In 2013, I ran 17:58 for the 5K, 37:57 for the 10K, and 1:22:37 for the half-marathon.

In early 2016, however, I tore my left calf trying to squeeze a speed workout into a lunch break, and it's been an uphill battle from thereon in. The last four years have been a mixture of niggles, minor injuries (Achilles, knee, glute), interspersed with a few half-decent times (I ran 18:48 for 5K and 39:02 for 10KM last May, and 1:29:12 for the half last March). Broadly, however, it's been increasingly difficult to even stay afloat, let alone improve.

However, winter brought another set of niggles, and now I'm at a point where they're lifting - but I really don't know how I can train in a sustainable way, and here is where I was hoping for some advice.

Problem 1: Consistency

I'm struggling to work out how many miles I should be aiming for each week. The last four weeks have seen me run 20km, 36km, 31km, 11km. Should I be aiming for three runs a week now? Four times? Is 30km a week a sustainable benchmark to begin with, or is that likely to accelerate injury risk?

Problem 2: Intensity

It's really difficult to know how much speedwork is too much here. At the moment I've been aiming to do 400m repeats once a week, which seems fine, and I can often do something like 85/86 seconds on each lap for 8-10 repeats, but it also seems clear that I should add some tempo running to my weekly plan. Is it sustainable to do a tempo in the same week as the speed workouts, or should I alternate? Put otherwise, which of these is preferable?

Tempo - Easy - Easy - Long Run
Speed Form - Easy - Tempo - Long Run

Or should I be foregoing all speedwork given the staccato nature of my training, and just focus on hitting mileage targets? If so, for how long should I do so?

Problem 3: Target Pace

It's difficult to know what a reasonable benchmark is, given everything. This year my quickest recorded 5K is 19:44, on 4th January - should this be my benchmark for training, or is four months now too long ago? Is it foolish to adopt a 1:30 half-marathon training plan based on this - if so, would 1:35 or 1:40 be more reasonable?

Any insight or advice would be greatly appreciated. I'm essentially just desperate to get back to a point where I can train sustainably and injury-free again, but feel a bit lost - 17:58 feels a world away. I'm also adding things like eccentric heel drops and glute strengthening exercises to my weekly regimen, too.