I think everyone suffers this when they first start using Trainz.
What I would suggest is deciding on paper what initial industries you want on your route (not too many. Just 2 or 3 pairs that make sense to the route).
Then build some basic trackwork between the industries.
I generally like to travel between 5 or 10 minutes from one industry to the corresponding one, so I use this as a gauge.
Once you've got the basic layout, choose the main town area and start decorating it.
More importantly, name it; give it an identity.
Once you've done a fair amount of work on a named area, you'll feel like driving around there, and to destinations from there, and it will become more important to you.
After driving from your main area to an industry out of town, you'll find you want to arrive at a more finished location, rather than just track and industry, so do it!
Another thing to remember is Do Not Rush!
Try to enjoy the experience of slowly bringing it to life, and resist the urge to start another project.
If you start to get bored building on your route, leave Trainz alone for a day or two, and come back to your route refreshed and with new ideas.
Remember, it's easy to start a dozen layouts, but it's difficult to finish one, so do yourself a favour and start just one.
That way, you're ahead of the game from the start.
And finally, use craft, not a million assets!
If you pile all the assets you can into your layout, not only will it suffer in performance, but it will also look unrealistic, and you'll notice this when driving it.
Take a train journey and look at how similar it all looks as you travel between towns and cities, but more importantly, there will be subtle little things you notice on your journey (a man walking his dog, a trolley in a stream, a tree blown over by the wind, a fisherman under his brolley, etc)
There's not a simple answer to your question, but I've found that if you enjoy test-driving your route, then you're more likely to finish it.
Smiley.