That's a very interesting difference. I find that being psychologically prepared to do something productive makes it way easier. If I trick myself into going for a swim by telling the lazy part of myself that I'll just go for 20 minutes, you can bet that if 20 minutes is up and I try to motivate myself to keep going, there will be several sub-components of my mind screaming 'but you promised!' This is even more true for things that aren't habits for me. (Exercise is something I do pretty easily by pure habit.)