Depending on the weather, I would try to keep them alive in the water until you are lamost ready to start back. Clean them just before you start back.
If the weather is colder than that water, clean them immediately, and hang. You can wrap them in a burlap bag. If you keep the bag wet, evaporation will further chill them.
Refrigerators are kept below 40 for a reason. And from experience I've found that keeping ours at 34 F (1.5 C) increases the shelf life a lot. (E.g. at 34 F keeping hamburger for a couple weeks isn't a problem) At 40, its good for about 4 days.
The spoilage rate is first order exponenential from above freezing to about 120 F. What will keep for 4 days at 40 will keep 1 day at 50 will keep 6 hours at 60 will keep 2 hours at 70. Will I swear to those numbers? No. Much depends on the bacteria count at the start.
It may be to your advantage to use an edible anticeptic on the fish immediately after cleaning it. E.g. 2 oz of Jack Daniels in a gallon of water. This will reduce surface bacteria and may double the time you have until it spoils.
The other option is to bring a collapsible cooler. They aren't as effective. You still have the hassle of ice.
If the weather is cool, I wouldn't worry about it. If the weather is hot, let the fish be until you come up with a portable cooler.
In passing: If the trail is a good trail, a bicycle makes a great carrying system. Don't ride it, just strap what you want to carry, and roll the bicycle. Viet Cong used to move 600 lb loads this way.