Historically, I have only used the elliptical as cross training when injured. An injury (posterior tibial tendinitis and a possible related stress reaction) knocked me out of Boston this year about 5 weeks before the race when I was in really good shape (but that may have been a blessing in disguise given the weather). Despite the injury I signed up for Chicago to try to capitalize on my existing fitness, as I don't see myself running too many more marathons and the longer I wait, the less likely I am to hit some time goals that I have in my mind since I am already 46.
So between the injury, and the challenge of training for a fall marathon through the summer in Florida, I decided to make the elliptical a more integral part of my training this cycle. When training for the marathon, I am a big believer in getting in as many 10 mile/70 minute days as possible. So for the most part, my elliptical time was usually integrated with a run to produce a (nearly) continuous 70 minute or more aerobic session rather than to use it in place of a double session - but that is the result of preference for singles over doubles.
From time to time, I would just use the elliptical in place of a run altogether. Most times when I did that, it was for a duration of 45 minutes or so, but in a few instance it was 60-75 minutes. I have also found that I can simulate tempo/threshold efforts pretty well on the elliptical, but I only did that once this training cycle.
Below is a week by week list of the number of 7-minute increments I spent on the elliptical each week for the 14 weeks ending as of last week. It averages out to about seven 7-minute increments per week, but with a fair amount of week-to-week variation. During this same 14 week stretch, I averaged 62 miles per week of actual running, so that should put things in perspective - maybe about 10%of my total aerobic volume came via the elliptical, but with a lot of variety from week to week.
9
5
13
6
0
0
11
0
4
7
10
9
0
25
0