Running - a range of distances

As I lost my superfluous fat, at least 10kg of it, and changed my running technique from heel strike to midfoot landing, I transformed as a runner.
Although I gained half the weight now again mostly in muscle mass, I do feel very lightfooted.
At the track, I can now stay close to the talented teenagers, I am 34, and until now a non-runner.
The way I've been improving though, forces me to retract my own words. I no longer am a non-runner. For that, I keep up with the real runner a bit too easily. Running technique, for over 30 years I wouldn't believe more than a percent or so could be gotten from it. Now, I know I just scooped up about 10% or raw speed, on technique alone. Can''t imagine having to use thick heel super-damped shoes to accomodate my heel striking technique again. Let alone trying to do a run at any pace that way.

Most of the time, I've been getting 2 track practice session in per week. much less in January, and still not good in February, due to having lost muscle strength to run the way I do now. Skiing got me soft. Now, I'm trying to increase my training volume. Through one or 2 endurance runs (or what I use the term for), and added short SkiErg sessions.

Yesterday, I somehow managed despite sore legs from 2 previous days of running, to set training PB's on 3 distances : 400, 600 and 1000m. Helped by having relatively long 4' rest between each. 64', 1'45 and 3'19. The former was relatively quick, the latter ralatively slow. Yet, the short stuff awazes me of late. I can really stay close to the quick teenagers there.
I ran those on my softer road shoes even, Adizero Rockets, not even on spikes.
Which race times belong to such practice times, run solo mostly in lane 2?

Our coach tells me that, especially when I turn 35+ Master next season, I would actuall be best at 100 through 400m. Oddest thing, but possibly true. Old guys get slow, and I just get started yet I'm not.

My aim with running, really is to get to a good 3000m time. It's a mix of endurance power, and anaerobic speed. Should get me in decent shape for a Winter Triathlon's opening running leg, being 6km or more, hilly, over snow cover.
I only have a time at 5km right now. Between last Oct and Dec I ran 3 of those, increasingly better. 18'00 my mark as of now. I want to get that down to 16'00, to be equal to ~9'00 over 3000m. Some way to go, obviously.

This summer season I'll be gathering courage to attend some track races. Over 1000, 1500, 3000 and maybe even 5000m. Shorter I may try once I feel safe in terms of injuring myself. On the road, I will try to get in some 5's, 10's and maybe even a 15km. Next up, a 5km in the program of the great Rotterdam Marathon. I wrote down 17'00 but wonder if that's doable.

Over 5km, 1kg of body weight more of less (let's presume it useless fat), is worth 11s of time lost or gained. I'm a bit heavier now that when I ran the 18'00, but I should also have developped as a runner, and gained from slihtly increased weekly cardio load.
My coach wants me to run a 1000m in 2'50 this season, and a 1500m in 4'40, to prove that I'm on track towards my running goals. I've never had to give quite that much in a race, so I'm almost scared to experience it. Afraid to just crash through my legs from exhaustion.

Being a run-sprinter, makes me wonder if maybe I am also a short-distance guy for skiing? I'm tall as are some sprinters (1m94/6'4", ~88kg now), but was never the guy to reach the highest top speed, let alone in the last meters. My best distance is probably an extended sprint. In MTB racing, 15-30mins seemed good for me, too bad races were over an hour usually.

Endurance can be trained, speed less so I think, so I'll try to consider my relatively decent speed a gift. Now, to learn to hold 72' laps for 7.5 laps on end...