You should start excising atleast a week before you start playing. Since its been so long since you played, your knees and muscles would be a little rusty. You might want to stretch them out. Don't just start playing one fine day, trust me, you will have to endure a lot of pain and discomfort after the game.
- Take it slow
Start by getting up in the morning and walking a few miles. Then start jogging. Get those knees and muscles to stretch. Do a few basic exercises, like a few push-ups and crutches. Stretch your arms and do a bit of arm and neck rotations.
- Start playing for short periods
Don't play for a very long time. Play for 10-15 minutes for you initial game. You can gradually increase the duration once your body adjusts to the action.
- Never forget to re-hydrate
Your body will tend to sweat out a lot and you could be severely dehydrated. So drink water and have electrolytes.
- Talk to your team and your opponents
Being careful is always a good option. Don't hesitate to tell you team and your opponents to take it easy as you have not played for a long time. You don't want them sliding for the ball at your feet.
- Always stay safe
Don't forget your shin guard and your basic protections. Wear good boots and proper jersey material.
- Don't hesitate to call it a day
You should stop playing whenever you don't feel all that good. Don't push your body to play longer. Your body might not have enough juice to go on with the game any longer.
These tips should help you. Always consult an expert, say a football coach closer to your age or a football physician. Stay safe, stay active. And there's nothing better than football.