Tom and I agree!!
The science of running can get very technical, but lets look at some high level theories. Lets take a distance runner who will be in training for several months. The goal will be to train without incurring injury and to peak towards the end of the season when top performance times are needed.
There are 3 phases to the training season:
Base - which is long runs (endurance training) at a slower pace.
Lactate Training - usually 1 - 2 months running further than race distance at a pace about 15-20 seconds less than 5K race pace (tempos). These are not done every day, and more than on tempo is done.
Intervals - previously discussed.
Tempos and intervals can have similar benefits, but tempos can deteriorate the body, because there are no rest periods as in intervals. Also the length of the tempo or interval are dependent upon the distance the runner will be racing. Middle distance runners usually have shorter intervals than 5K or 10K runners.
For most distance runners, the intervals are called speed work because this stage fine tunes the athletes performance for those important races and the end of the season when time counts.
And during the entire season there is usually some base (endurance training) in each stage. Even during the interval stage runners will do some base days and some tempo days.
Regarding the attitude needed for intervals. This depends on the individual. I tend to be a task oriented person. When I ran track 40 years ago, I enjoyed 440 yd intervals more than any other training. I did 65 - 70 second intervals depending upon the number of intervals. I enjoyed the precise distance and timing on the track.
But most of us are not athletes, so I really question the value of interval training for most of us. Now if we just do not much time to exercise every day, then there can be some benefit to interval training, if one starts out slow and is careful. Even mechanical equipment can cause injuries.
As Craig pointed out, swimming is great exercise and can be low impact... if you have access to a pool. I had a roommate in college who was a nationally ranked butterfly swimmer and he was in phenomenal shape. He could do 70 push ups in 60 seconds.
So what is best for the average backpacker here on BPL? For me I try to do a hike every weekend, whether it is a day hike with my wife or a solo over-night trip. During the week when I have to work, I try to do a minimum of 1 hour per day at the gym. 30 minutes on any elliptical and 30 minutes on several stations for varied muscle groups. When time permits, I run a few miles. The weight training are light weights with 3 sets of 15 or 20 reps. Sometimes I run and go to the gym each day. But none of the training is "intense" intervals. This routine also requires that I eat more than most people my age. Before I started this exercise routine a few years ago, I ate a lot less and maintained my weight. Unfortunately, the past two months I have not been doing the weekly exercise, but have been doing day hikes on weekends. Since I have not adjusted my caloric intake, I have gained around 20 lbs, and suddenly weigh more than I ever have. So, this week I just started making time to get back into my exercise routine.