Average speed doesn't tell anyone much. If you spent the whole mission at 11 knots, you'd probably last about 3 hours. But if you spend some time at flank and some time at 2 knots such that the average speed was 11, you'd be lucky to last much longer than an hour or so. The way it's set up is for an exponential increase in battery drain as speed increases; what you end up with is that up until about 5 knots you can plod along indefinitely, up to about 10 knots you have a significant drain rate that will press you after a few hours, and anything beyond 12 knots or so will blow your battery's load like it was a nervous teenager on prom night.
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