"I had to start very specific fitness preparations, I started in February, so now I'm 100 per cent," he said. "We did a couple of fitness tests 15 days ago and I had the best results in my career, so I'm extremely motivated, happy and stronger than ever."
However, because of rule changes teams will use the same cars for 2021 and Alonso may not have a genuinely competitive car until 2022, when he will turn 41 in late July.
"I will try to do my best, and try and help the team be a world champion team. If that's with me driving, fantastic but if that's with a future younger driver, I will feel proud anyway," he said.
"I think the 2022 rules will hopefully bring some fairness to the sport and some close action with teams more level and less scope to invent something that has a large performance advantage."
Alonso, who will race alongside French driver Esteban Ocon next year, had quit F1 to focus on winning motorsport's triple crown. Having already won the Monaco Grand Prix, he then won the 24 Hours of Le Mans endurance race, but not the Indianapolis 500.
Alonso won 32 F1 races and has 97 top-three finishes.
- AP