Two hour run

Today I did a 2 hour run along the Sumidagawa. It was the first time in about 4 months that I've been able to run along the river without making detours to avoid closed parts of the river path. All the work the council was doing has now been finished, so I can run for an hour in one direction without leaving the path.
Today I ran with a heart rate monitor on and tried to hold 170 beats per minute - quite a task. I ended up averaging 164 beats per minute - which was OK, because I stopped a couple of times to buy drinks (it was really humid).
I know I need to increase my endurance. I lose a lot of time when I run a 5km because I can't maintain the same pace longer than 4km. I think the 2 hour runs are just what I need - and they will make it easier to start training for the Tokyo marathon, which I really must focus on soon. I want to break 3 hours - but my best time is 3.31, so I'm not sure I can improve by such a big amount so quickly.