The road begins before you think it does
The mistake most people make is treating Manali as a starting line. You arrive, sleep one night in a guesthouse off the Mall Road, and assume the body will sort itself out once the wheels start turning. It won't. The acclimatization that matters happens in the 48 hours before you leave, not on the road itself. Drink water until you're tired of it. Eat less than you think you need. Walk slowly. The Himalayan altitude is not punishing you for being unfit, it's indifferent to your fitness. A marathon runner and a sedentary office worker can both get acute mountain sickness at 3,500 metres. The body's adjustment to thin air has nothing to do with how strong your legs are.
