Maybe it's the offseason; it's winter; you're injured; it just started raining; it's dark; your fitness has plateaued; it's early; you're dehydrated; it's late; you just had a bad race; your last workout went poorly; your gear is worn out; it's cold; you're tired; you should be stronger; you don't stand a chance against the competition; work is stressful; you're traveling; it's the holidays; life is wearing you down; your muscles are sore; you're hungry; the race is months away.
You just don't feel like it.
There are endless reasons to not train, to not do a planned workout, and to lose sight of the lofty goals you set for yourself. With endurance racing, goals often take years to accomplish, sometimes decades. Maintaining motivation over those kinds of time periods is extremely difficult.
You need a game plan, though. Something you will automatically go to when the excuses come flying into your head. What will you do to combat them when they come? Being ready is crucial to surviving them, and getting out the door.
Remember what your goal is. What are you trying to accomplish? Talk yourself through it before every workout. You're down in the weeds doing nitty gritty workouts and it may be hard to see the big picture, so step back and remember the big picture. This is about the race in June. It might be cold now and tough getting out the door, but it's going to be far worse if you do poorly on the race you put so much effort into.
Each workout is a building block. Keep that in mind, and remember to evaluate if you are building with the right blocks. If you're confident you've got the right approach, and that the completion of the workouts will get you your result, you will be much less likely to question it when things get tough. You can just put your head down, and get it done without thinking. 400 workouts might get you in good shape for your race, but those extra 15 workouts you "didn't feel like doing" would have given you the win, instead of the 5th place you got. You're already going 95%, don't shirk the last 5% and forfeit the result and goal you truly want.
They are working hard, too. They aren't taking the day off; they are heading out into the rain to get it done. They are doing that last set of drills. They are trying to beat you NOW.
There are very few parts of a race that are actually decided on race day. Most of it can be easily predicted by the preparation of the contenders. Your competition? They are training when you're "not motivated," they're putting in the work you're not. Doing that extra interval. Pushing past their limits. Hitting that workout harder than ever before.
Everyone trains on the nice days. Use the tough days as the days you get your edge. When you do what no one else would consider. When you get that leg up, the extra work in, the intervals that hurt, the workouts people save for the best of days. Do it on the hardest days, and you'll be ahead, ready, primed. Race day will be easy.
Remember what you're shooting for, and all the pieces it takes to get there. Then do the work, even when it sucks, remembering that if you beat yourself today, you WILL crush them tomorrow.