If you don't have any undisclosed health issues it should be no problem. As already mentioned by another, it's not a race. Or is that something you have not disclosed to us.
Essentially you just need to figure out what amount of leg muscle you can use and not tire out completely. The more you ride the more muscle you can use without tiring. But what that is will depend entirely on you.
When I'm riding, still without a PM. I maintain pretty much what I think is a constant power output to my legs. I don't try to maintain a certain speed at all unless I'm with a group and speed is what the group goes by.
For solo or when I'm in front of others that don't care about speed, it's my perceived power that dictates things, not speed. I know what level of perceived effort will let me ride all day and what level will only get me to the top of a hill.