The best way to get in shape is to just get out and hike! Start light and add weight.
Health club alternatives include the infamous stair stepper and the treadmill (angled at 10-15 degrees and with a pack on your back is sure to get everyone looking at you strangely). Actually, any of the aerobic machines will help, but concentrate on strengthening those quads (the fronts of your thighs). You can strengthen with various types of squats and leg extensions. I suggest that you vary the weight and repititions. If you lack strength, consider the "Body for Life" regimen: 12 reps at a very easy weight, 60 seconds of rest, 10 reps at a weight that let's you know you're lifting, rest for 1 minute, 8 reps at a weight where you could do 3-4 reps if you had to, rest for 1 minute, 6 reps at a heavy weight where you can only punch out one more rep (using good form, of course), rest a minute, 12 reps as heavy as you can, then, without resting 12 reps of an exercise that works the same muscle group as heavy as you can. You can use this for any muscle group. Give your muscles 2-3 days to rest before repeating.