So you've probably trained for a running event before, probably trained for a swimming event or a triathlon before. The key difference with swimrun racing is that you will be racing with a partner* in unusual attire, in the water, out the water, repeat.
Train with your partner if you can. If you can't then not to worry, you'll just have to treat the first third of your swimrun race as training... this can go well, and it can go very wrong.
The second thing to get your head around is running in your wetsuit - it's perfectly OK to cut down an old thin wetsuit and race in this. Although the first time you cut your wetsuit it feels like sacrilege, it is how many swimrunners start. You may want to invest in a swimrun specific wetsuit further down the line, among other things these have the advantage of having the zip on the front. There are many run sections that you will want to unzip your wetsuit and roll it down to the waist. The easier it is to do this the faster your transition will be. We can recommend the HEAD Aero which we reviewed here.
So wetsuit sorted - put it on and go for a run!
Swimming in trainers is no drama. It's just like swimming, except you've got trainers on. Using a pull buoy to increase buoyancy helps. Check out our article on gear to see how to modify your pull buoy. Note when in and out of water your laces (if you use them) can magically untie so make sure you triple knot them.
Next up is to try swimming, transitioning and running with a tow system. Coming from a kayaking and white water perspective I was initially very reluctant to tether myself to someone else in the water with lots of other potentially crazy swimmers! The old "water and ropes don't mix" adage is forever present. It's a bit of a fixed mindset adage... swimrun however is undoubtedly a growth mindset endeavour.
Firstly you won't be using a rope you'll be using a bungee. Secondly you will use a clip/karabiner that should break given significant forces. Thirdly your waist-belt will be releasable (if you use a tow-float waste belt). So in summary don't worry about being tethered at all. You won't realise how effective it is until you try it, I'll be writing an article on the benefits of towing in swimrun (and how it can be used in swim coaching) soon.
Something that might catch newcomers out is you get given a race bib to race in. This makes unzipping and peeling down your wetsuit slightly more awkward, and will add a little extra drag on your swim.
Try all of the above whilst wearing an old race t-shirt to mimic the bib.
Forgetting about the kit for a moment, remember the basics - hill training is possibly the most effective run training, so work it into your training schedule. Swimming ten to twenty sets of 100m to 250m at 80% is a simple way to improve your swimming, and a few swim coaching sessions to improve technique is usually well worth the investment.
One thing I did before Coniston was to write down the 8 swim distances and complete them in the pool (resulting in a great 5km swim session). After each swim distance I pulled myself out of the deep end had a cold shower and jumped back in, just to mark the end/start of each "transition".
If you can get some swim - run - swim - run sessions in your training then that's ideal. It might be worth a weekend in Wales/Lake District/Scotland/suitable coastline to facilitate this.