Agreed that this is not a race. Don't worry about the time. Just have a good time.
To conserve some energy try riding the first 40-50 miles in a slightly easier gear than you usually ride in. It worked for me when I did my first metric.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.