Your body doesn't want to be worked hard. Your brain will use any trick in the book to avoid it. I'm not a runner, but I remember reading an article about running where it said something like "The biggest step you will ever take is the one across your front door."
What you need to do is build exercise of some kind into your routine. It needs to become as much part of you as eating or going to the toilet. Along time ago I went to the gym 2-3 times a week. I had my ritual, packed my bag in advance, picked it up and just went. I didn't think about whether I wanted to or not. It invariably felt good afterwards though. I had a fortnight off over the Christmas period at one point and somehow never went back.
I began to cycle much later and thought I had learned some lessons from that gym experience. I kept it going for 3-4 years then had a combination of things that rocked me, including being hit by a car. I kept some of it up, including a very short daily commute, but this summer I've begun to build up again and have had to be honest with myself about how much my fitness had slipped from a peak 3 years ago.
There are still things that help. Packing my gym bag the night before definitely increases my chance of going early the next morning. I'm more of a fair weather cyclist than I was. I'm on a very flexible work pattern and a couple of times recentlty we've had very nice days. I've packed up early, gone home and got out on my bike. That has brought a huge sense of exhilaration. The whole routine has been part of it. Looking out of the window, seeing the sun, telling my colleagues I'm leaving early to go out on my bike, switching off my computer, have all led inevitably to that bike ride being very pleasurable because of the build up as well as the bike.
Don't just think about cycling. I walk dog, go to the gym (where I sit on an exercise bike) and mostly cycle in dry conditions now. Have a target for exercise which is a little ahead of where you are. I used to take my car about 800 metres to buy a sandwich at work. Stopping that and deciding to walk every day was a significant change at the time. It was quickly overtaken by other changes but it was a key part in getting me started.
Good luck - let us know how you are getting on!