4 km/h on average, less in Paris, more during floods. If he swims four hours a day, that’s 16 km or 10 miles. That’s comparable to a good swimmer.

That’s why this part felt interesting:

> if he could maintain an average of 10 miles a day, he would need just over 50 days to complete

It’s not very warm, so I’d definitely exercise to keep some heat but I don’t know how much of the feat was drifting.