Do our bodies tire out at a "constant rate"?

What i mean is, I've recently started using a treadmill at 7.5 km/h for 10 minutes (does that suck? but I've just started) and that's about the time I start panting. I know that the greater the speed, the greater the energy I'm consuming, so that must mean that I won't be consuming as much energy if I were to jog at 4km/h for ten minutes. Therefore I'd be able to run for much longer at 4 km/h, but for how long? For ten minutes at 7.5 km/h, that's really 1.25 km and if a relationship were linear, then my body MUST be able to run at 4 km/h for about 19 minutes and 15 seconds to acheive that same distance of 1.25km, right? Or does the body not conserve energy at a steady rate and I could possibly either tire out before or after that calculated time? Whatever, I'll find out on my next run.