To be honest you might struggle with an 18 inch net. An average fish would be say 25 inches or more on most rivers, I know you can net them head or tail first using a net smaller than the fish but on your own it is quite tricky. And if you get a big un then you will have no chance (saw one caught 42 inches last week

) I usually don't carry a net as the beat I fish has a lot of gravel banks that are easy to beach a fish on, although the other day I hooked a fish on the only pool where you can't beach a fish and had to tail it by hand in about a foot of water - not easy on your own.
If the beat you are fishing is mostly gravel banks then I'd beach them as a first choice. Take you net as well just in case. If the beat had steep banks and no gravel I'd look at maybe getting/borrowing a bigger net.