Kayaking alone is a bit of a contentious issue for most, but assuming you’re paddling well within your limits, have at least two means of attracting attention if you’re in trouble, and have made sure to tell someone where you’re going and when you’ll be back, it’s generally ok - at the end of the day, the only person you might harm would be yourself!
Anyway, that’s besides your question:
When paddling solo, some people will cycle or run the shuttle (depending on how far or how difficult the terrain is to do this), other times you can attempt to hitch hike (usually more successful at the start of the day when you’re dry!) - either way, it’s always best to make sure your car is at the bottom so you have warm clothes available as soon as you finish, so best to drop your boat and paddles somewhere hidden or secure at the top, drive to the bottom and then do whatever you’ve got to do to get back to the start!
This question represents a real problem for kayakers !!!!
This question represents a real problem for kayakers, and I’m glad you asked it. the most common solution, (bike back to the launch after parking at the takeout), which is my personal favorite solution. I’ve done this myself, numerous times, when doing relatively short (+/- 10 miles) paddles.
Another option, especially for popular rivers, is to check out the local guides/outfitters. Odds are, there is some company out there willing to shuttle you back to the launch, for a nominal fee. You can also check social media, to see if that area has a paddling group, and if so, reach out to the group to see if anyone wants to join you, or, if anyone will shuttle you.
Good luck, and paddle on!
Experience a Friend
You may have to invest in a kayak cart. You can then pull your kayak or even attach it to your bicycle.
I use my kayak cart to access Western Brook Gorge in Gros Morne National Park (Western Newfoundland). There is no road, but there is a 3 km foot path. That adds up to 6 km of walking, but it’s a breeze when you balance the cart wheels under your kayak properly. The kayak is then no heavier than a briefcase and you can walk all day with it in tow.