I listen to a podcast, No Sugar No Grains, and they interview many guests who are long distance athletes (runners, ultra runners, tri's) who fuel without sugars. I am on an injury break right now, but I have done a few training rounds for half marathons while eating low carb. I would train by only fueling on fats and then on race time take some sugar to get that extra fire. I can't remember which book I read it in (Noakes or Perlmutter?) but if you train on keto/low carb but trickle in sugar during a race, you get a big burst.
When I did the 2014 WDW marathon, my food fell out of my belt right at the beginning. I don't know where the food stations were but I did not get anything to eat until after 10 miles. Then of course, everything on the race course is sugar. I am a slow runner, so I finished in 6:00 and I remember feeling pain from being on my feet that long, not passing out due to lack of sugar.
If you are truly following a keto/low carb diet, you will do just fine. Snack ideas are olives, packets or baggies of nuts or a scoop of nut butter, cheese cubes (if you are ok with dairy), or UCANN starch. Also, if you are running you may find that you can afford to have more carbs in your diet. On my running days, I could have up to 100 carbs a day and still be in ketosis. But remember that those carbs should be from vegetables.