I Am A Non-diabetic. My Recent A1C Level Was 5.9,

I am a non-diabetic. My recent A1C level was 5.9, so my dr started me on an 1800 calorie diet and increased exercise. Recently, when I walk ( 3 miles), I start to feel shaky at the end of my walk. Prior to walking, I ate a protein bar and celery and peanut butter. I still felt shaky. So, I tested my blood sugar 15 minutes after my walk ended and my result was 77. I thought exercise increased your blood sugar? Is there something else I should eat prior to walking my 3 mile route?