I used to be a runner, switched 100% to cycling for several years, then switched back to running (a few days a week at first, cycling the other days, then eventually 95% to running).
The first year after switching back, the cycling training was apparently pretty helpful. I weighed 20-30 pounds more than in peak running fitness, ran very close to my mile PR (in trainers!), but I couldn't run a great half marathon.
So when I first started running, I built some running-specific fitness and had all of this residual cycling fitness to back it up. But as I backed off the cycling, it became harder and harder to build my running back up to where it was before and my overall aerobic workload dropped off a ton. The first year after resuming running I improved rapidly at first, but my second year my times got worse as I lost that residual fitness from cycling and struggled to build running mileage. Now I'm in my third or fourth year after cycling, my mileage has increased and my workout consistency has improved, and my running level has finally risen again to the highest level it's been since switching back from cycling.
Regarding Gwen Jorgensen, I'm wondering if she's going to struggle to maintain her fitness from running alone. She's got this huge aerobic engine and a lot of muscle, developed by working out several times a day for many hours a day, and it might be tough to find ways to maintain that as a runner. In fact she might not want to maintain all of it (the upper body for sure), but some of it is functional. So I wouldn't be surprised if she did really well in her first season or year of running, and then we saw a dip in her performance before it ultimately improved.