If the Traditional IRA amount was from contributions of pre-tax dollars, then the conversion to a Roth IRA which is made up of after tax dollars will be taxed at your normal income tax rate. The the distributions taken in later years will be tax free withdrawals.
If you find that your tax is going to be more than you can cover, then you could get in touch with your plan custodian and request a recharacterization of the full amount or partial amount.
