No, you will have to register again for the 2019 edition of 100 Days of Running using the new registration process which will go live on Jan 26th, 2019.
However, if you specify the same email address for registering for the 2019 edition which you used for past edition, you will be able to login and use the previously setup account on the 100 Days of Running website after registering for 2019 edition.
Without registering for 2019 edition, you will not be able to participate in the same, even though your login/account may exist from a past edition of the event.