They don't make you stop the ride you're on, so you could get 2 rides up Alpe du Zwift or Ven-top if you really wanted to get your hours out of that 25 km.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"