One of the things about travel is that often we will have to do a lot more physical activity that we might normally do. After all, most of the best views are at the top and most historical sites don't have elevators.
Time: 3 weeks minimum
Somewhere to exercise
Walking and hiking
Usually when you are a tourist you will find yourself walking around most cities. You want to start walking regularly around your local area. Maybe walk to the shops or if you take the bus to work, why not walk to the next bus stop along. At work, go for a walk during your lunch break.
Hills and stairs
Most tourist locations are at the top of stairs, hills and even mountains. The best way to build up to this is walk every hill in the area. Ideally you will have stairs you can take as well as they use different muscles to gentle slopes. If you live in an apartment building then you can take the stairs. Step classes at the gym can also really help.
Lifting and carrying
It is likely that you will have to manhandle your luggage to some degree. Though even if you do everything first class there is the chance that you might have to still do this.
I was once stuck in a snow storm in Beijing. My luggage was very heavy and taxis were really hard to come by. So I had to lug my luggage through the snow to the subway station, then down the stairs and then up the stairs at the other end.
You want to exercise to build up your lifting strength, so this includes your legs as well as your biceps. It is a good idea to also do shoulder shrugs in the gym as well. Not only does carrying luggage put strain on your shoulders, if you are carrying a day backpack it rests on your shoulders as well.
You also want to be able to lift your luggage into overhead compartments so work out for that as well.
Chances if you are going somewhere warm you will end up on the beach or at the pool. You are going to have more fun if you have swum at least a few laps before your holiday.
If are an adventure tourist you could find yourself doing everything from kayaking rivers to climbing up cliffs. Usually it is a good idea to try these things at home first before heading abroad.
Don't spoil your holiday
Your holiday can be limited or ruined because you weren't physically fit enough for the activities you have planned. So next time you go on holiday spend a few weeks getting fit so you can fully enjoy it.