No, the chunk of your loan repayments that goes towards interest changes over time.
This is called "amortisation".
With amortisation, part of each monthly repayment goes towards the interest on your loan. The rest is used to pay off the capital (the money you received).
Your first monthly Zopa repayment consists of a higher proportion of interest and a lower proportion of capital.
As you continue to pay off your loan, a progressively larger chunk of each repayment goes towards paying off the capital.
You'll often find this process in the world of finance, since it allows customers to repay a loan in predictable fixed instalments each month.