I nearly got to 10K last year. Our local training group started at 1km and added 1km a week. the 9km week had more tide than expected so I ended up with 3:30 in the water with 1 kayak support for 10 swimmers so if you waved and waited you got a drink and a kitkat.
I was also in a wetsuit.
If you treat the swim as a walk on the hills then I think you'll be able to complete the swim from a fitness perspective. I found the biggest challenge to be the saltwater after 3 hours. It began to feel like I was stuffing my face with salty crisps every breath. Up till that point I was really enjoying it.
No idea what the channel swimmers do about this. Rinse more often with clean drinking water perhaps.
Have a great swim whatever and good luck with your challenge.