If you're really not confident about the motorway, then you are better off taking longer and taking roads you feel okay about - also, see what the conditions are like on the day. If it's clear and dry, then consider the motorway. If it's lousing it down and visibility is reduced, then stick to roads you feel more confident on, because it's just an extra factor you'll need to consider.
When I passed my test, I then didn't drive for about 7 years, mostly because I couldn't afford to, though my parents insisted I did when I was at theirs, to keep my hand in. When I finally got a car, I took a couple of refresher lessons, one of which was a motorway lesson - not least because I grew up in the country and took my test in a small town, and simply hadn't had much practice on big roads. Also, having spent most of the previous few years walking or cycling, it took me a little while to find speeds over 30mph not too scary. So if you don't often travel on big roads where you are at the national speed limit, it will seem that much faster than it would if you're used to nipping up and down a dual carriageway. If you can do a dual carriageway, a motorway is fine.
It really was fine - the driving instructor talked me through it, and pointed out all the traffic was going in the same direction, so it's actually easier than driving on A-roads. They are safer roads overall. She made me drive back home on A-roads. Overtaking on an A-road is far more nerve-racking than on a motorway. (I'm not suggesting that you do any overtaking on any sort of road, unless you're confident about it, and the road is clear.) But it did make a big difference, having someone experienced with me and talking me through it that first time.
I later spent some years commuting a route that was partly on the motorway, and I'm now fine with it. If it's a route I don't know, then I will spend some time before the journey checking which junctions I need. Also, there's no shame in going on to the next junction and coming back if you miss the one you need - and anyway, you won't have a choice! (However, there is sometimes a long distance between junctions, so best avoided if you can.) If I've got a long journey, I will also work out where I can take breaks.
But do the journey you're comfortable with. If you've time to practice on shorter journeys, then do that. But don't push yourself to do something you're not ready for.