Discussion: Sustainable for how long?

For beach running, that is a fairly brisk pace.

I hadn’t given any thought to how my pace might change on a different surface, like a hard packed trail.

I haven’t really set a duration target beyond being able to run on trails between checkpoints in a 60-90 minute Orienteering event. I think I would miss cutoff times with some frequency if I walked the whole course. I’d like to run if I knew where I was going and walk if I was less certain.

So...I envision running 4-7 minutes. 2-3 minutes looking for a CP and resetting my bearings then repeating for 55-90 minutes. I’m ok completing all races with the slowest non DNF.

Beach running is probably similar to a trail in terms of unreliable footing but with no hills. Lots of orienteers and trail runners walk up hills so you won't look out of place if you do that. I don't think you need to go any faster than you're already going; the next step would be to go at a lower HR and aim for 2 km. Typical 90-minute orienteering courses range from 4-8 km so you'll get *way* more bang for your buck from accurate navigation, which is one of the things I love about the sport. Physical power is helpful but brain power will reduce the amount of physical power you need.

I was planning to continue with the strict 1km limit until I got 10 runs in, then checking what my heart was running at - it feels like 145-155, but I’m not sure how that translates from bike to foot.

Then I was going to do some combination of:
a) testing what pace keeps me below 145 and maybe 140
b) experiment with repeat intervals - I feel like I could run a kilometre again within 5 minutes. That’s mostly walking off my calves. My heart feels back to normal in a minute (without having timed it).

I think if I can gain 15minutes over walking, I can finish a course on time. Then see if I can gain enough navigation experience over a race series to gain another 15-30 minutes with good decision making - and maybe get down to double the winning time.

Sounds like a fine plan. About the only thing I can add is that people typically test at a higher max HR for running vs. biking (and lower for swimming). I'm not sure how that fits with your unique needs though.

My expectation is that the venous deficiency will show sooner running. Clipped in on a bike, I can keep my calves pumping. I am intrigued by the possibility that with the right cadence and stride, I might be able to run farther than I can walk.