The casting, despite being the most obvious, isn't the only thing that needs practice. I also like to spend a bit of time getting my retrieves where I want them. Probably the main thing is speed, especially for mahi mahi as an extra turn of speed will often turn a chaser into an eater.If the winds are low and drift speeds allow it's not a problem to do this, but offshore fishing in the Sagami Gulf usually involves a bit of a breeze and I've found that if I don't practise my rolypoly retrieve I sometimes can't move the fly quickly enough. A sweep of the rod sometimes works but bending the straight line makes for poorer hooking.
For the tuna species around here the speed it seems is perhaps less important than smoothness of the retrieve, something I also need to tune back in before fishing both for rolypoly and figure 8 retrieves, you might be surprised just how fast and smooth you can get a figure 8 retrieve up to with enough practise. Fortunately it's easy to combine this with casting down the park or the lake, but I do like to be deliberate rather than just rattling the line back for the next cast.
The other option is to do it at home, specially for figure 8 style, I'll often set up the bottom half of a rod and repeatedly retrieve a full line back and forth gradually increasing the length of the vertical stroke between the twist of the hand at various speeds. If it becomes jerky in any way I'll drop back on the speed or stroke length and focus on getting the motion to flow smoothly again before slowly speeding up.
I imagine that probably fewer people practise their retrieves than their casting, but it's probably not surprising as a lot of applications don't really warrant it. I picked up the habit as a teenager fishing loch style competitions where the rules require the rod to be held in the hand on the retrieve, but I don't think you need the constraint of rules to get some benefit from working on retrieves, just a drifting boat or a fish that swims faster than you can retrieve.