This was not a satisfactory answer for my young mind. I continued to wonder why they did this. In my spiritual immaturity (have a become mature even know?) it seemed like a silly thing to do. Though I now see this was likely also colored heavily by my Protestant/Evangelical upbringing. In addition to fish, some gave up Coke, others ice cream, and so on with a list of dietary options. What I failed to understand then, as I wonder today if many still do, is that the practice of giving something up, is that you may fill your life with something else, focussed on the things of God. So, say trading TV between 7-9 pm for reading theology, scripture, praying, journalling etc. The Lenten exercises are to focus not exclusively on subtraction but upon addition (look at that...math talk from an artist!).
This year, as I have a number of times in the past, my Lenten addition is praying the hours as found in Phyllis Tickle's Eastertide. The hours include 4 prayers each day (morning, noon, early evening, and before bed). Tickle infuses the prayers with standard collects from the liturgy, lots of Psalms and Gospel readings. It is a good place to begin with prayer of the hours. These prayers are extracted from the larger 4 volume set that runs according to the seasons. I would highly recommend them all.