Probably not. Overtraining is specific to the individual exerciser, and your body will tell you it's getting too much physical activity by prolonged feelings of fatigue, worsened sleep patterns, lack of enthusiasm for your exercise sessions, weird appetite (often depressed appetite), feelings of overall irritability, and other symptoms.
If you're on your feet for most of those 12 hours on duty (and, just as an aside, there's a pair of angel's wings for a nurse - I could NEVER do what nurses do!), I'd pay attention to making sure you have proper footwear (hopefully your employer is sensitive to that in its dress code policy), as well as making sure ALL of the lower body and core muscles are stretched out (calves, hamstrings, quadriceps, hip flexors and hip extensors {glutes}, and erector spinae). Prolonged standing and walking is enormous endurance work.
Let your body be your guide in terms of training - my guess is, from what you've described, you've developed a very workable workout protocol for you.
A-Jock
