John, many studies are showing that interval training (as Jonathan pointed out; aerobic) can have great health benefits for seniors. That does not negate steady state, both are great components of a fitness program. With any high risk client I would get doctor permission first. These are some studies that talk about HIIT with high risk populations that I like to show hesitant doctors:
Bartlett, J.D. et al. (2011). High-intensity interval running is perceived to be more enjoyable than moderate-intensity continuous exercise: implications for exercise adherence. Journal of Sports Sciences, 29, 6, 547-553.
Guiraud, T. et al. (2012). High-intensity interval training in cardiac rehabilitation. Sports Medicine, 42, 7, 587-605.
Kessler, H., Sisson, S. and Short, K. (2012). The potential for high-intensity interval training to reduce cardiometabolic disease risk. Sports Medicine, 42, 6, 489-509.
Tjonnas, A.E., Lee SJ, Rogonmo O et al. (2008). Aerobic interval training vs. cContinuous moderate exercise as a treatment for the metabolic sy├¡ndrome: Aa pilot study. Circulation, 2008;doi:10.116/circulationaha.108.772822
I personally have my clients do at least 2 months of steady state before I throw in intervals. Then I do so extremely slowly; starting with 10 seconds of high intensity and 2 minutes of recovery, with only 3 intervals at first. (For my high risk clients). Then I very gradually (and only one at a time) increase the high intensity interval and decrease the recovery. They only do this 2 days a week, with strength and steady state training on other days. My clients have had great health results (decreased blood sugar and blood pressure as well as weight loss) with such a program.