Here's what I've discovered over the years: efficient gym usage can play an important role in fastpacking, but the key word is "efficient." Aimlessly wandering around a gym is not the best use of your time. Efficiency is extremely important for people with active and busy lives, what with work, kids, and events vying for your precious time. I've always tried to do the minimal amount of work that will enable me to accomplish a specific goal. I cover many of these points with a bit more depth in Backpacking Strategies: Hiking More than 20 Miles Per Day but would like to talk about what I'm doing right now, and that's preparing for a sub 36 hour Wonderland Trail run sometime this coming fall.
Work the Basics: As we age we lose muscle mass....perhaps as much as 1% per year after age 40 (without exercising). The older I get, the more important it is for me to maintain muscle and tone, so I always incorporate some type of lifting into my routine. When I say basics I'm talking about the larger groups that sometimes incorporate whole body movements. Core is essential, so I spend probably half my lifting time working my core, utilizing a 10 pound medicine ball and doing multiple sets of 20 over 10 different exercises with no rest in between. By June I will repeat this set 3 times, for a total of 600 reps. Then I work the push muscles, the pull muscles, the lift 'em up muscles and the legs. You won't see me doing exercises targeting specific muscles, like rotators or biceps; I'll leave that to the body builders or people who want to look pretty. I want utility; being pretty is something I was born with! You also won't see me standing around resting; I'll go from one exercise to another, alternating the targeted muscle groups so I'm not hitting the same ones in a row. I'm no trainer so I won't offer routines. However, I'm looking for muscle endurance along with strength so I usually do sets of 20.
Maintain a Fitness Base: With the crappy weather prevalent in the Pacific Northwest during the winter, I don't get on the trail nearly enough. I've found that maintaining a base through treadmill running and (Lifecycle) biking allows me to ramp up for hiking season efficiently. My worst year was 2009, but I still managed a 3 day Wonderland, albeit a painful one. In my program (by the end of June) I will be running 10-15 miles (but usually no more than 2 hours) on the treadmill twice a week and biking 30-50 miles (usually no more than 3 hours) at least once.
Cyclical is OK: Notice I'm not obsessing about hitting the gym 6 days a week. If I can avoid going more than 3 days without working out, I'm good. Trying to maintain peak fitness throughout the year is extremely difficult, both mentally and physically. For me, ramping up for the summer and fall treats my body well, allowing for long periods of rest. IT IS BETTER TO UNDERTRAIN THAN OVERTRAIN! REST IS THE MOST IMPORTANT PART OF ANY FITNESS PLAN! INJURY WILL DERAIL A FITNESS REGIMEN MORE THAN ANYTHING ELSE! These are 3 of my personal rules.
Untuck Your Shirt: Just because you're old doesn't mean you have to look the part. Ditch the magazine while you're biking and just get into the workout. Arrange your lifting so you're done quickly, you should be sweating, breathing a bit with an elevated heart rate just like an aerobic workout....because it is!
Visit the Gym Throughout the Summer: Sure I'll be doing long trail runs, bike rides and backpacks as I shock my system into going hour after hour, rediscovering the agony of flopping legs but exercising the mental toughness to get through it (because you can always recover!). But I still need those core sessions and treadmill runs to keep minimal strength and clean out the junk. It's hard looking out the window on a nice day, but we can't always spare the time to hit the trail.
Balance is Paramount: If you dread working out, then something is probably amiss. I've found cruising through the winter with minimal but essential base training in the gym allows me to turn up the heat, both mentally and physically, along with the improving weather of spring. By the time I'm on the trail in the summer I'm eagerly absorbing every breath, sight and sound. If I've timed my fitness routine right I'll have the legs, will and stamina to pull off whatever my goal is. And by the end of the hiking season I'll be ready to give my body a break with basic gym sessions to keep atrophy at bay. Works for me.