At the gym (where I've retreated while waiting for the glaciers outside to recede) I struggle to do a half hour on a stationary bike at the cadence/speeds that on my actual bike I can do for 3 hours without stopping. Any ideas why? or more to the point, any ideas on how to improve? -- Bad positioning of seat/pedals/handlebars? -- Machine actually underreports how hard I'm working? -- I'm just out of shape for cycling (though been doing other aerobic/strengthening stuff regularly all winter)?