The stamina message means "your racer couldn't reach her top speed" while guts means "she ran out of stamina completely" which is accompanied by being close to last.
Generally speaking, first you want to meet the stamina/guts requirement for the distance otherwise you're not going to reach the maximum speed during the final spurt -which is when everyone says fuck positioning and starts sprinting- or worse, you'll drastically slow down after hitting 0 stam. Guts determines how fast stamina drains during this and iirc uphills as well.
HOWEVER you have +400 to stats during the campaign which makes it easy to clear races but makes you shit the bed for anything under the "race" tab.
Also people get golden stamina recovery skills which are equivalent to 3 common skills and scale off stamina which makes guts not as desirable (so you'd click the responses to events that give you stamina and avoid the ones that give you guts)
Only after you have enough stamina, go with speed.
You can get away with no stamina for sprints and maybe mile, ~400 for medium, while the longest 3.2km race needs about 600 (or ~1k for team trials)
The "not enough power" message means your racer had shit positioning and got boxed in next to the rail by 6 turtles until she had an opening. You can avoid it by running as front (for debut/early career even if you have an F for frontrunner it will work, aptitude for that affects decision making and positioning ability) or another strategy that goes further ahead but has decent aptitude.
Also there's a small chance for your frontrunner to get a very late start that you can never recover from which is why 'Focus' is so popular.
