The thought of cycling vs actually being out there

I’m fairly new to cycling as a means of exercise (I’m currently using my father’s bike and I only really considered it as a consequence of a running injury).

One thing I’ve noticed however is how massively different my perspective on it is when I’m sat doing nothing in comparison to actually being on the bike. As I type this the thought of going out for a 40km ride fills me with dread, but whenever I’m actually out there I really enjoy it. Is this common for newbies?