Build up gradually
Start small. Do a few shorter walks, say 5-10 km, a couple of times a week. Then gradually up it by around 10-15% each week. Aim for longer walks (15-20 km) on the weekends.

Start small. Do a few shorter walks, say 5-10 km, a couple of times a week. Then gradually up it by around 10-15% each week. Aim for longer walks (15-20 km) on the weekends.
Chuck in some strength training. Think squats, lunges, and planks to beef up your legs, hips, and core.
Give your legs a break by mixing in some cycling or swimming. It's easier on the joints and keeps your cardio ticking along nicely.
Don't forget the stretches. Do dynamic ones before walking and static stretches after. Focus on tight spots like your calves, hamstrings, and hips. A foam roller could be your new best friend!
Kit yourself out with decent, worn-in walking shoes and some moisture-wicking socks. Make sure you've tested all your gear (clothes, hydration pack, etc.) during your training.
Drink plenty of water before, during, and after your walks. Bring a bottle or some electrolyte drinks for the longer treks. Practice munching on snacks like energy bars or fruit while you're walking.
Try training on similar terrain to the actual 25km walk. If it's got hills, get some incline in during training.
Make sure you're taking rest days. Your muscles need time to recover.
Keep your head in the game. Mental stamina's just as crucial. Try mindfulness, get some tunes going, or better yet, have a chat with a mate while walking to stay motivated.