Correct inflation is a bit tricky until you learn how to manage it. If you can get on of the smaller hand pumps and keep it in the side pocket of the tube you can adjust on the fly, so to speak. I frequently transport mine in the back of my pick up about 15 miles then spend about 30 minutes with it on my back in the sunshine hiking to my put in point, I tend to keep it firm for the temp but allow for solar expansion. When I put it in the river with its 50 deg water I may need to add a little air at my first stop if its not good and firm when I launch. Then when I take it out I let out a little air to allow for expansion. But I have a smaller pump I keep in a dry bag (my FishCat has an area behind the seat you can stick things). Keep you inlet/outlets tight, pump it up at home and see if it leaks, you could have a small leak which are easy to do and easy to repair. I have never had a tube deflate to where I was in danger but a sagging tube is not much fun.
"Patriotism supporting your country all of the time and and your government when it deserves it"- Mark Twain