Struggle Bus but Workout Complete

Boy was this morning's workout a STRUGGLE. I was supposed to do a HIIT workout (which I have to modify normally because of my awesome knees) but I wasn't feeling that and my right knee has been talking to me recently. So I picked a level 3 cardio workout which on a normal day would have been fine. Today it was a serious struggle. I don't know if it is because I didn't eat enough or because I have a head cold. I'm taking my normal rest day tomorrow and plan on changing up my pre-workout food for Saturday morning. I've been just eating a banana when I get up, waiting about 20 mins and then working out. I then eat my normal healthy breakfast afterwards. Maybe I need to beef up my pre-workout food and wait a little longer?