Guys, get it straight
Ingrid Kristiansen is THE model to follow when it comes to running and cross training
Ingrid ran more than Parker ever has
Ingrid also cross trained more than Parker ever has
Ingrid also simultaneously held the WRs in the 5000, 10000 and Marathon
Cross-training can be extremely beneficial, but the truth is there are only one or two legit options: XC-Skiing and to a lesser extent the arc trainer
I believe one of the biggest issues runners have with cross-training is they don't understand how much more volume the body is capable of when doing non-impact cardio, but also they tend not to understand (or maybe consider) that because they only cross-train when injured it means their body is not able to put in as much volume as would be needed to sustain or increase fitness
In general, the body seems to be able to sustain no more than about 12 hrs/wk of running and no more than about 30 hrs/wk of non-impact cardio
It's a fairly simple equation after that to figure out how much more non-impact cardio a runner could theoretically handle beyond their current volume of running
Example: Runner A can handle 6 hrs/wk of running then tends to get hurt historically if they try to increase beyond that. For Runner A to max out their potential in terms of aerobic output per week, they'd need to build up to roughly 15 hrs/wk of non-impact cardio (XC-Skiing preferable if available, they do make summer skis with wheels too though never tried them)
Prob one of the biggest mental barriers runners have the sheer amount of volume the body can handle when doing non-impact. It takes some adjustment and ultimately most people don't have the time or physiological makeup to suffer in isolation that long day after day. Granted, half the people reading this would do almost anything to be able to workout longer just because that's the best part of their day.