@sky13 if you do have access to a stationary bike, this is what I've learned from personal experience thus far.
1) Make sure you're doing a structured routine on the bike. I use TrainerRoad and highly recommend it. I've seen far better performance management/gains using a structured routine than simply getting on the bike and putting in time.
2) The effects on the heart are different between running and cycling. Running increase the size of the left ventricular which in turn increases stroke volume. Your heart can beat less and transport the same amount of blood/oxygen throughout the body (more efficient). It's a critical adaptation to becoming faster/maintaining running performance. I've seen personal evidence in my resting HR data and running Garmin VO2max data.
Evidenced in this graph which displays my running Garmin VO2max value (black line) and resting HR data (blue line):
View attachment 437141
In periods of high running activity, my resting HR falls. In periods of high cycling activity in the absence of running, my resting HR rises. It wasn't until running was added back in did my resting HR drop back down again. I believe this is be evidence of the increase in stroke volume and efficiency of the heart muscle.
3) From my personal data, it would appear the adding in cycling as a replacement of running in order to yield the same level of running performance would follow this formula for me:
Take normal amount of running
Subtract out the amount of running you can no longer do
In order to have the same level of fitness with run+bike+strength you need to do 2x the duration you use to run.
Example:
-I use to average 7 hours of running per week.
-I now average 4 hours of running per week.
-In order to have the same level of running fitness as when I did 7 hours of running per week, I now need to do 14 hours of run+cycle+strength. So that would work out to 4 hours running + 10 additional hours of structured cycling and strength work.
The succinct conclusion here is that running and cycling are not 1:1. A 60 min run can't be replaced by a 60 min cycle to yield similar running results over the long term.