Where did this straight line come from, at what stage - before or after the walk?? I thought I had done everything to avoid that. I assume there is no way to turn the tracking mode on/off as on the old Etrex Venture?
Probably after the walk, if you switched on the GPS at home within an hour of switching off (in which case MM will append the current position to the original track). But it's not too difficult (within MM on PC) to right-click on the start of the straight line, select "split track" and delete the unwanted line.
I don't know the GPS62s but I'm surprised if it doesn't have a "simulate" (GPS receiver off) or similar mode. But if not, I'm afraid it's Garmin to blame.