How much is the time or heart rate difference between passenger/no passenger?
Don't know! The bike path isn't perfectly flat and it's in this steep little valley that makes the gps tracking generally really poor, so it's not really clear how different in speed... and I'm not sure I know where my HR monitor strap is... Definitely is some added work though - I'd guess it sort of helps in the way running uphill helps? Need more oomph with each stride to make forward progress?
Great resistance training ... with no stroller you'll feel (and be) super-speedy!